> That's assuming you finance your property with debt and let someone else pay for your debt. If you either happen to own a property you don't use, or own the property you live in, it makes perfect sense.
If you happen to own a property, but not use it, then you can either:
a) rent it out, or
b) sell it, and invest your proceeds according to your preferred risk/reward preferences.
In an efficient free market, for similar risk you get similar reward; So, if the reward for real-estate is 3% with essentially no risk, you're generally able to get the same thing by selling and putting the money in bank / low risk stock portfolio.
Alternatively, if you can't get 3% in other places, everyone will look to buy houses and rent them out (as an investment) in order to get those 3%. That's why the risk/reward profile equalizes across asset classes.
What GP was describing is basic math and economics. The existing state of ownership only matters in the sense that changing it introduces economic friction (ownership deduction, property transfer taxes, title transfer, realtor and lawyer fees, etc). Other than that, it's just math.